In most radio frequency (RF) communications receivers, it is necessary to mix the RF signal to a low frequency intermediate frequency (IF) signal to perform signal processing and demodulation. This function is performed by a down mixer which uses well known heterodyne principals to mix a RF signal to some lower frequency. Typically in modern day communications equipment, this is accomplished with either a Gilbert Cell mixer or a diode mixer.
As seen in prior art FIGS. 1 and 2, a Gilbert Cell mixer is illustrated wherein an RF input signal 101 is fed to a RF input transconductor amplifier 103 such as a differential pair. The RF signal is amplified and then input to a mixer 105 formed by a plurality of switching devices. The mixer 105 works to provide sum and difference RF signals based on the frequency of the input signals from RF input transconductor amplifier 103 and the local oscillator (LO) input signals 107. With proper application of a differential load 109 and the voltage supply 111, the resultant output signals are provided at the output 113.
Although the typical Gilbert mixer 150 shown in prior art FIG. 2 is commonly used, this circuit performs the mixing function at some cost and inefficiency. Namely, the noise figure of this type of mixer is typically very high and is commonly between 8 dB to 10 dB at 800 Mhz. This high noise figure occurs because the RF Input Transconductor amplifier 103 does not have a substantially large power gain to overcome the noise figure of the switching devices. This limited gain is due to the large impedance mismatch between the RF transconductor amplifier 103 and the switch devices present in the mixer 105. As a result, the mixer 105 must use a low noise amplifier (LNA) in a stage preceding the mixer which ultimately consumes more power and degrades the third order input intercept point of the mixer 105. In the instance where this circuit is used as a RF receiver front end, it creates more receiver noise and higher current drain contributing to the overall inefficiency of the circuit. Typical prior art mixer circuits are disclosed in U.S. Pat. No. 5,884,154 and U.S. Pat. No 4,636,663 and are both herein incorporated by reference.
Thus, the need exists to provide a RF mixer topology that has a substantially low noise figure and with a proportionally large power gain and low current drain for eliminating the need for an LNA commonly used in RF mixer circuitry.